(a) Field of the Invention
The present invention relates to a zoom lens. More specifically, the present invention relates to a miniature zoom lens for cameras using an image pickup device such as a charge-coupled device (CCD).
(b) Description of the Related Art
Recently, the use of electronic still cameras and video cameras employing a CCD or a solid-state image pickup device has been expanding rapidly. Cameras are being built into mobile phones, so the needs for miniaturization and weight/cost savings are increasing. Miniaturization and weight/cost savings are also needed for zoom lenses that are built into cameras.
The implementation methods for a zoom lens include a method of properly arranging a small number of lenses to provide a small zoom lens with high image quality, and a method of using plastic lenses to realize a low-cost zoom lens.
The conventional optical system realized by the former method is disclosed in Japanese Patent Application Laid-Open No. 1998-213745. The zoom lens disclosed in this cited patent comprises, in order from an object side: a first lens group entirely having a negative refractive power with one or two negative lenses and one positive lens; a second lens group entirely having a positive refractive power with one positive lens and one negative lens; and a third lens group entirely having a positive refractive power with at most two lenses. This zoom lens is characterized in that the third lens group is moved with a convex locus on the image side during zooming from the wide-angle position to the telephoto position. The focal length of the first lens group divided by the total focal length at the wide-angle position is between 2.5 and 4.0. The second lens group is comprised of a lens having a strong positive refractive power and a meniscus lens having a negative refractive power so as to control the spherical aberration and astigmatism, and the meniscus negative lens is used to eliminate the chromatic aberration.
However, the zoom lens has a problem in regard to a great displacement of the third lens group, so it is difficult to realize the miniaturization of the zoom lens.
On the other hand, the conventional optical systems employing the latter method are disclosed in Japanese Patent Application Laid-Open Nos. 1997-21950 and 2000-267009. Among those conventional techniques, the zoom lens disclosed in Japanese Patent Application Laid-Open No. 1997-21950 includes, in order from an object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. Each of the first and second lens groups includes at least three lenses of which at least two lenses are plastic lenses. But, this zoom lens uses a large number of lenses to increase the size of the optical system and it becomes difficult to realize a low-cost optical system.
The conventional optical systems using a solid-state image pickup device are as follows:                (1) Japanese Patent Application Laid-Open No. Sho 56-123512        (2) Japanese Patent Application Laid-Open No. Sho 63-292106        (3) Japanese Patent Application Laid-Open No. Pyung 6-94996        (4) Japanese Patent Application Laid-Open No. Pyung 5-173071        (5) Japanese Patent Application Laid-Open No. Pyung 11-84243        
The zoom lens disclosed in the cited patent (1) comprises, in order from an object side, a first lens group having a negative refractive power, and a second lens group having a positive refractive power. The second lens group is moved to perform zooming, and the first lens group is moved to compensate for the focus movement caused by the zooming. But, the first and second lens groups are moved along the optical axis to perform zooming, so the over-all length of the optical system is greatly varied during zooming. Therefore, the construction of the camera barrel is too complex, and it becomes difficult to realize a zoom ratio of at least 2× and particularly to realize miniaturization of the zoom lens.
The zoom lens disclosed in the cited patent (2) is a lens system of which the over-all length is not varied during zooming. The zoom lens comprises, in order from an object side, a first lens group having a negative refractive power that is fixed during zooming, a second lens group having a positive refractive power that is moved along the optical axis during zooming, and a third lens group having a positive refractive power. The second and third lens groups are moved towards the object side during zooming from the wide-angle position to the telephoto position. The over-all length of the optical system is required to be long enough so as to correct the performance at a wide angle, resulting in a large-sized optical system. Therefore, it becomes difficult to realize miniaturization of the zoom lens.
The zoom lens disclosed in the cited patent (3) is a three-group type zoom lens that includes a third lens group having a negative or positive refractive power on the image side of a second lens group to correct all kinds of aberrations, so as to enhance the zoom ratio and make the entire lens system compact in a two-group type zooming system. In this zoom lens, the first and second lens groups are moved along the optical axis to perform zooming, and the third lens group is fixed. But, the over-all length of the optical system is varied and the zoom ratio is at most 2×.
The zoom lens disclosed in the cited patent (4) comprises, in order from an object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, a third lens group having a negative refractive power, and a fourth lens group having a positive refractive power. The zoom lens has such a structure that the respective lens groups are moved during zooming, so the configuration of the camera barrel is too complex.
The zoom lens disclosed in the cited patent (5) comprises, in order from an object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. During zooming, the distance between the first and second lens groups and the distance between the second and third lens groups are decreasing. In this structure, the second and third lens groups are getting closer to each other at the wide-angle position, so the focusing according to the distance from the object must be performed with the first or second lens group. The first and second lens groups have a much larger number of lenses than in the third lens group. The number of lenses to be moved during focusing is thus increased to increase the focusing error, and the moving part of the lens system is extremely large in volume. Therefore, it becomes difficult to realize miniaturization of the zoom lens.